To deal rationally with the complexity of present communication systems and with the need to make different systems mutually compatible, the International Standards Organization ("ISO") developed a model for specifying such systems. Using this model, called the Open Systems Interconnect ("OSI") model, a communication system can be broken down into a hierarchial structure that permits standards to be defined at each level in the structure. The OSI model provides a hierarchy of seven different layers that can occur in a communication system. Each layer in the OSI model covers a different function performed by the communication system.
The lowest layer in the OSI model, called the physical layer, specifies the physical structure of interfaces in a particular communication system or network. Thus, a standard for the physical layer of a communication system specifies such things as the number of wires, their electrical characteristics, the characteristics of signals transmitted over the wires, connectors used for joining two sets of wires into a single longer set of wires, etc.
The next higher layer in the OSI model, called the data link layer, specifies how data is transmitted error free through the communication system. Thus, a standard for the second layer in the OSI model specifies how to detect errors in transmissions passing over the physical layer, and how to correct any errors that may occur during transmission.
The next higher layer in the OSI model, called the network layer, specifies the manner in which connections are formed between various places in the communication system for transmitting data between them. The standard for the third layer in the OSI model, therefore, specifies the signals transmitted over the data link layer that cause the communication system to transfer data between two places on the network.
A standard defined by an International Telegraph and Telephone Consultative Committee ("CCITT") for the ISDN communication channel specifies these three lowest levels in the OSI model. Under the CCITT standard, a basic ISDN access consists of two full-duplex 64 kilobits per second ("kbps") digital data channels, called channel B1 and channel B2, plus another full-duplex 16-kbps digital channel, called a D channel. Under the CCITT standard, using time division multiplexing, all three of these digital data channels may be transmitted over a single pair of twisted wires, or over two pairs of twisted wires. ISDN basic access, as specified by CCITT, was originally intended to provide a basic digital data transmission capability suitable for use by individuals such as in their homes or small businesses.
When ISDN basic access was initially specified, each of the B channels was intended to carry either:
1. digital data, such as that from a personal computer or from a computer terminal;
2. Pulse Code Modulation ("PCM") encoded digital voice communication; or
3. a mixture of lower data rate communications including digital data and digitized voice that were each encoded at a fraction of each B channel's full 64-kbps capacity.
Under the ISDN specification, the D channel serves two purposes. First, the D channel carries signaling information that controls the transmission of data over the two B channels. In addition, when the D channel is not carrying signaling information, it may be used to transmit packet-switching or low-speed telemetry. The combined data rate at which digital data may be transmitted over twisted pairs of wires in accordance with the ISDN standard for basic access is 144-kbps, i.e. 128-kbps for the combined B1 and B2 channels plus 16-kbps for the D channel.
In addition to the ISDN basic access specified by CCITT, that organization has also specified a higher performance ISDN communication channel identified as ISDN primary access. An ISDN primary access provides twenty three 64-kbps B channels plus one 16-kbps D channel for a total capacity of approximately 1.5 megabits per second ("mbps"). CCITT envisions that ISDN primary access can be used for communications between an ISDN local exchange and an ISDN Private Branch Exchange ("PBX"). Moreover, a recent development in ISDN communication technology is directed toward providing a service called "bandwidth on demand" in which an ISDN primary access communication channel is not fixed at 23 B channels plus one D channel, but can vary in capacity from instant to instant depending upon the transitory need for communication capacity.
Because the CCITT standard for the ISDN communication channel specifies the lowest three layers of the OSI model, the ISDN standard provides interfaces, both physical, e.g., the plug in a wall, and logical, e.g., electrical signals passing through the plug. In achieving this result, the ISDN standard specifies several different physical interfaces, the most widespread of which is called the S interface. The S interface of the ISDN standard specifies the interface between Terminal Equipment ("TE"), e.g., a telephone, and a Network Termination ("NT") of the ISDN communication channel.
In North America, the S interface is the four wires usually found in a home telephone installation. In this interface, two of the four wires transmit data from the Network Termination to the Terminal Equipment, and two wires transmit data back from the TE to the NT. That is, the NT uses one pair of the four wires to transmit the combined B1, B2 and D channels of ISDN basic access to the TE, while the TE simultaneously transmits a different combined B1, B2 and D channels back to the NT on a different pair of the four wires.
While ISDN basic access was originally intended to provide voice and slow speed data communication services such as those identified above, over the years developments in digital signal processing and compression techniques have advanced technology to the extent that compressed video data may now be transmitted using ISDN basic access. These techniques have progressed to such an extent that there now exist several alternative video data compression techniques such as the CCITT H.261 picture phone standard, the Joint Photographic Experts Group ("JPEG") standard, and the Motion Picture Experts Group ("MPEG") standard.
U.S. Pat. No. 5,027,400, that issued Jun. 25, 1991, on an application filed in the names of Toru Baji et al. ("the Baji et al. patent"), discloses a multimedia bidirectional broadcasting system that distributes motion picture data using a broadband ISDN communication channel. In the system depicted in FIG. 3 of the Baji et al. patent, a motion picture program data base is maintained at a broadcasting station for transmission over broadband ISDN communication channels in response to requests received at the broadcasting station from subscriber systems. In the broadcasting station disclosed in the Baji et al. patent, an image encoder compresses a video signal prior to its transmission over the broadband ISDN communication channel to the subscriber system. The subscriber system includes a decoder for decoding the compressed video data and a television monitor for displaying them. Both the broadcasting stations and the subscriber systems disclosed in the Baji et al. patent transmit and receive video data compressed in accordance with a single compression standard.
FIGS. 1-1 through 2-1, 3, 4 13, 15, 20, 28, 32 and 35 of the Baji et al. patent disclose various different configurations for the subscriber system. FIG. 1-6 depicts a subscriber system adapted for use in a video mail application. In the video mail application, the subscriber system depicted in FIG. 1-6 transmits compressed video data back to the broadcasting station for storage there and subsequent re-transmission to a different subscriber system.
While various subscriber systems disclosed in the Baji et al. patent include a "graphic processor 141," it is capable of only simple screen operations such as the shift and drag of an icon. The graphic processor 141 disclosed in the Baji et al. patent is incapable of "realtime number crunching" required for more sophisticated screen operations. Consequently, to provide enhanced graphic capability at the subscriber system of the Baji et al. patent, the broadcasting station, such as that illustrated in FIG. 1-4, may include an image processing engine 187, for effecting time-consuming 3-dimensional graphics processing and the like, and various accelerators (processors) 188. Located in the broadcasting station, the image processing engine 187 and the accelerators 188 are time shared among many subscriber systems communicating with the broadcasting station.
In addition to the broadcasting station disclosed in the Baji et al. patent, a PCT patent application entitled "Adaptive Video File Server and Methods for Its Use," filed Feb. 11, 1992 in the names of Mark C. Koz and Masato Hata, discloses a video file server or broadcasting station that includes both a random access data storage subsystem and an archive data storage subsystem for storing compressed video data. In response to commands from subscriber systems, the video file server of this PCT patent application transmits compressed video data to the subscriber systems over communication lines, or receives compressed video data therefrom in accordance with a variety of different compression standards. Compression-decompression cards included in the video file server allow it to adaptively transmit and receive compressed video data compressed in accordance with a variety of different compression standards. The compression-decompression cards of the video file server also provide it with an authoring capability for storing compressed video and/or audio data in the random access data storage subsystem and/or archive data storage subsystem. This PCT patent application disclosing a video file server is incorporated herein by reference.